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Active Tectonics: Studies in Geophysics 13 Dating Methods KENNETH L.PIERCE U.S. Geological Survey, Denver ABSTRACT Geologic assessment of active tectonism depends on two key measures: the age and the amount of deformation of a given stratigraphic unit. The amount of deformation can normally be measured with greater accuracy than the age. Adequate age control is thus a limiting factor in studies of active tectonism. About 26 dating techniques can be applied to dating deposits and deformation of late Cenozoic age (past few million years). These techniques can be grouped as numerical, relative dating, and correlation. Numerical techniques are best, but datable materials are often lacking, and in these cases age estimation must be made using relative-dating or correlation techniques. Relative-dating techniques are nearly always applicable but are not precise and require calibration. Correlation techniques are locally useful and depend on recognition of an event whose age is known, such as a volcanic eruption or a paleomagnetic reversal. Geologic studies of active tectonism are greatly aided by definition and time calibration of local stratigraphic sequences. Because all dating techniques may be subject to considerable error, reliability should be assessed by stratigraphic consistency between results of different dating methods or of the same method. Numerical ages may have large, nonanalytical errors. For example, radiocarbon dating is one of the most useful techniques in the 0–50-ka range (ka= thousand years old), but contamination can result in dates as young as 15 ka for deposits that should be isotopically dead (>40–70 ka). Improvements in our ability to date active tectonism and define its rates will come from continued refinement of established techniques, such as carbon-14 dating, and development of experimental techniques, of which thermolumineseence seems to offer special promise. Experimental isotopic-dating techniques such as 10Be, 36Cl, and 26Al also offer potential for dating faulting and other deformation. However, refinement of relative-dating techniques, such as soil development, rock weathering, and progressive landform modification are likely, because of their general applicability, to provide much of the needed age control. For a given fault or other feature of active tectonism, deformation rates need to be determined over different time spans to recognize any variations in deformation rate and how attempts at prediction of future deformation may relate to these variations. If grouping of faulting events has occurred, hazard assessment based on deformation rates depends on the combination of the pattern of deformation and the time window of observation.
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Active Tectonics: Studies in Geophysics INTRODUCTION Dating is a critical tool in the assessment of active tectonism. The two primary measures of active tectonism are the age and the amount of deformation of a stratigraphic unit, which together define rates of deformation. Although slip or other deformation rates may vary through time, such rates are still one of the most useful measures of active tectonism (Slemmons, 1977). Dating of active tectonism is commonly accomplished by dating surficial or volcanic deposits. In addition to dating conventional stratigraphic units, materials such as calcite infillings may be deposited in a fault plane and dating of these infillings used to determine the history of faulting. With either conventional stratigraphic units or materials infilling a fault zone, if the material is unbroken its age provides a minimum age for the last faulting; and if it is broken, it provides a maximum age for the last faulting. Although individual fault offsets obviously occur in jumps, the overall slip rate can be determined from the amount of offset, especially for multiple events, divided by the geologic time interval involved. In the past few decades, our ability to date geologically young deformation and associated deposits has improved greatly. In this time, dozens of dating techniques (Tables 13.1 and 13.2) have been either developed or greatly refined. As few as 10 yr ago, estimates of the age of Quaternary deposits were commonly in error by severalfold, and it was not uncommon for age estimates to have been off by a factor of 10. Although we have recently learned much about the ages of young deposits and deformation, we still have a long way to go, for one of the greatest constraints in our understanding active tectonism is accurate and reliable dating in order to define rates of past deformation and times of past earthquakes (Allen, Chapter 9, this volume). An appreciation of the amount of dating control needed is illustrated in Figure 13.1, which shows one model of fault activity through time. Predictions based on such a model require multiple dates to determine (1) the slope of the line shown as “accumulation rate,” (2) the interval between fault movements, and (3) the time since the last movement. To define a fault history involving multiple events, at least one age is needed to define each event. This kind of model depends on the assumption of a constant accumulation or slip rate; dating control spanning different time intervals is needed to know if the assumption of a constant slip rate is warranted. More than one age determination is required to establish reliable age control. Numerical ages are preferred, but relative-dating and correlation methods are important because they can provide age control in the absence FIGURE 13.1 Diagram of the earthquake-generation process of Cluff et al. (1980) showing importance of time (horizontal scale) in earthquake history and prediction. In order to predict the time of a future earthquake based on this model, note how much dating control is needed to define holding time, accumulation rate (similar to slip rate), and elapsed time, even for this example in which accumulation rate is assumed to be constant. of numerical techniques, or they can be used to evaluate the numerical ages, which can be subject to large nonanalytical errors. Surficial geologic studies and local time-calibrated stratigraphies are vital in the study of active tectonism, both to provide dating control and to evaluate the reliability of specific age estimates for tectonic events. OVERVIEW OF DATING METHODS About 26 different dating methods can be used in dating active tectonism. Table 13.1 categorizes these methods as primarily numerical, relative dating, or correlation (Table 13.1). The numerical methods are based on processes that do not require further calibration. Relative-dating methods are applied to a local sequence of deposits that differ in age but are similar in other characteristics, such as a sequence of glacial moraines or a flight of alluvial terrace deposits. Relative-dating methods provide information on the magnitude of age differences between stratigraphic units. If calibrated, relative-dating methods can be used to estimate numerical ages. This normally requires calibration by numerical methods and an understanding of the process being measured and its relevant history (e.g., temperature, precipitation). The correlation methods do not directly yield a numerical age, but if a feature can be correlated with an event whose age is known, such as a volcanic ash
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Active Tectonics: Studies in Geophysics TABLE 13.1 Classification of Methods Applicable to Dating Active Tectonisma Numerical Methods Relative Dating Methods Correlation Methods 1. Annual 2. Radiometric 3. Other Radiologic 4. Simple Process 5. Complex Processes 6. Correlation 1. Historical records 2. Dendrocronology 3. Varves 4. Carbon-14 5. Uranium series 6. Potassium-argon 7. Fission track 8. Uranium trend 9. Thermoluminescence and electron-spin resonance 10. Cosmogenic isotopes other than carbon-14 (10Be, 36Cl, 26Al, and others) 11. Amino-acid racemization 12. Obsidian hydration 13. Tephra hydration 14. Lichenometry 15. Soil development 16. Rock and mineral weathering 17. Progressive landform modification 18. Deposition rate 19. Geomorphic position and incision rate 20. Deformation rate 21. Stratigraphy 22. Tephrochronology 23. Paleomagnetism 24. Fossils and artifacts 25. Stable isotopes 26. Tektites and microtektites aAll methods listed are briefly described in Table 13.2. Methods given in italics are discussed in the text. eruption or a paleomagnetic reversal, precise age control can be obtained. A dating technique, whether it be primarily a numerical, relative-dating, or correlation method, may be converted to the other two categories of methods (Table 13.1). For example, the relative-dating methods of amino-acid racemization or soil development can also serve either as local correlation techniques or, if calibrated by numerical dating, as numerical techniques. Table 13.2 briefly summarizes 26 different dating methods noting their general applicability to studies of active tectonism, the age range of each method and the optimum accuracy within parts of this age range, and the basis of the method and the key problems in its use. The six columns (Table 13.1) are discussed in the next six sections. Beyond those given in Table 13.2, the criteria for selecting individual methods for discussion include at least two of the following: (1) the method is particularly applicable to dating active tectonics, (2) the method provides a good illustration of the six general categories (columns of Table 13.1), and (3) the method has complexities or problems that merit discussion. After this manuscript was completed, two books on Quaternary dating methods became available (Mahaney, 1984; Rutter, 1985). The reader interested in more extensive description and references to the literature may wish to consult these books. ANNUAL METHODS Annual methods (Table 13.1, column 1), generally accurate to the nearest year, provide the most precise dating of active tectonism. But excepting varve chronology, annual methods span too short a time interval to assess active tectonism, particularly in the western hemisphere. Only limited use has been made of dendrochronology and varve chronology. In the western hemisphere, historical records of faulting are restricted to about 200 yr (Bonilla, 1967). Based on the long historical records of seismicity from China (3000 yr) and Japan and the Middle East (2000 yr), Allen (1975, p. 1041) concluded that historical seismicity there shows “surprisingly large long-term temporal and spatial variations. The very short historical record in North America should, therefore, be used with extreme caution in estimating possible future seismic activity. The geologic history of late Quaternary faulting is the most promising source of statistics on frequency and location of large shocks.” RADIOMETRIC METHODS Because radiometric methods (Table 13.1, column 2) yield accurate numerical ages, samples for such dating are searched for in geologic studies of active tectonism. In many cases datable materials cannot be found. Additionally, radiometric methods may be subject to major errors and should be evaluated in their geologic context by other methods that, although not as precise, will normally be a valid indicator of the general age. Carbon-14 Dating Carbon-14 dating is generally the most precise and applicable numerical method for dating prehistoric faulting. Indeed, the chronology of the late Quaternary and particularly the Holocene (past 10 ka) is based on this method (for review, see Grootes, 1983). The analytical uncertainty is generally less than a few percent. Two applications of carbon-14 dating to active tec-
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Active Tectonics: Studies in Geophysics TABLE 13.2 Summary of Quaternary Dating Methods and Their Applicability to Dating Active Tectonism (from Colman and Pierce, 1979) Method Applicability Age Range and Optimum Resolution Basis of Method and Remarks 102 103 104 105 106 1. Historical records X to XXX Requires preservation of pertinent records; applicability depends on quality and detail of records. Limited to several hundred years in western hemisphere. 2. Dendrochronology XX Requires either direct counting of annual rings back from present or construction of a chronology based on variation in annual ring growth. Restricted to areas where trees of the required age and (or) environmental sensitivity are preserved. 3. Varve chronology X Requires either direct counting of varves back from present or construction of a chronology based on overlapping successions of continuous varved lake sediments. Subject to errors in matching separate sequences and to misidentification of annual layers. 4. Carbon-14 X to XXX Depends on availability of carbon. Based on decay of 14C, produced by cosmic radiation, to 14N. Subject to errors due to contamination, particularly in older deposits and in carbonate material (such as mollusk shells, marl, soil carbonate). See text. 5. Uranium series XX Used to date coral, mollusks, bone, cave carbonate, and carbonate coats on stones. Potentially useful in dating travertine and soil carbonate. A variety of isotopes of the U-decay series are used including 230Th/234U (most common and method described to left), 234U/238U (with a range back to 600,000 yr), 231Pa/235U (10,000–120,000 yr), U-He (0–2 m.y.), and 226Ra/230Th (<10,000 yr). Errors due to the lack of a closed chemical system are a common problem, especially in mollusks and bone. 6. Potassium-argon X Directly applicable only to igneous rocks and glauconite. Requires K-bearing phases such as feldspar, mica, and glass. Based on decay of 40K to 40Ar. Subject to errors due to excess argon, loss of argon, and contamination. 7. Fission track X Directly applicable only to igneous rocks (including volcanic ash); requires uranium-bearing material (zircon, sphene, apatite, glass). Based on the continuous accumulation of tracks (strained zones) caused by recoiling U fission products. Subject to errors due to track misidentification and to track annealing. 8. Uranium trend XXXX Based on open-system flux of uranium through sediment and soil; 238U, 234U, 230Th, and 232Th must be measured on about five different samples from a given aged deposit and a isochron constructed to determine age. 9. Thermoluminescence (TL) and electron-spin resonance (ESR) XXXX Based on displacement of electrons from parent atoms by alpha, beta, and gamma radiation. Applicable to feldspar and quartz in sediments and carbonate in soils. TL based on amount of light released as sample is heated compared with that released after known radiation dose. TL precision better than indicated for ceramics in 400–10,000-year range. 10. Cosmogenic isotopes other than carbon-14 X Dating methods analogous to 14C-dating are based on the cosmogenic isotopes (half-life in hyears in parentheses) 32Si (300), 41Ca (1.3×105), 36Cl (3.08×105), 26Al (7.3×105), 10Be (1.5× 106), 129I (1.6×107), and 53Mn (3.7×106). Dating requires knowledge of the generation rates, flux rates, and retention efficiency of the deposit dated. These radio-isotopes occur in very low abundances and are measured by accelerator mass spectrometry.
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Active Tectonics: Studies in Geophysics Method Applicability Age Range and Optimum Resolution Basis of Method and Remarks 102 103 104 105 106 11. Amino acid racemization XX Requires shell or skeletal material. Based on release of amino acids from protein and subsequent inversion of their stereoisomers. Shells tend to be more reliable than bone, wood, or organic-rich sediment. Is strongly dependent on other variables, especially temperature and leaching history. Commonly used as a relative dating or correlation technique, but yields numerical ages when calibrated by other techniques. 12. Obsidian hydration X Based on thickness of the hydrated layer along obsidian crack or surface formed during given event. Age proportional to the thickness squared. Calibration depends on experimental determination of hydration rate or numerical dating. Subject to errors due to temperature history and variation in chemical composition. 13. Tephra hydration X Requires volcanic ash. Based on the progressive filling of bubble cavities in glass shards with water. Subject to the same limits as obsidian hydration, plus others, including the geometry of ash shards and bubble cavities. 14. Lichenometry X to XXX Requires exposed, stable rock substrates suitable for lichen growth. Most common in alpine and arctic regions, where lichen thallus diameter is proportional to age. Subject to error due to climatic differences, lichen kill, and misidentification. The limit of the useful range varies considerably with climate and rock type. 15. Soil development XXXX Encompasses a number of soil properties that develop with time, all of which are dependent on other variables in addition to time (parent material, climate, vegetation, topography). Is most effective when these other variables are held constant or can be evaluated. Precision varies with the soil property measured; for example, accumulation of soil carbonate locally yields age estimates within ±20 percent. 16. Rock and mineral weathering XX Includes a number of rock and mineral-weathering features that develop with time, such as thickness of weathering rinds, solution of limestone, etching of pyroxene, grussification of granite, and buildup of desert varnish. Has the same basic limitations as soil development. Precision varies with the weathering feature measured. 17. Progressive landform modification XXX In addition to time, depends on factors such as climate and lithology. Depends on reconstruction of original landform and understanding of processes resulting in change of landform, including creep and erosion. 18. Rate of deposition XX Requires relatively constant rate of sedimentation over time intervals considered. Numerical ages based on sediment thickness between horizons dated by other methods. Quite variable in alluvial deposition. 19. Geomorphic position and incision rate XXX Geomorphic incision rates depend on stream size, sediment load, bedrock resistance to erosion, and uplift rates or other base-level changes. If one terrace level is dated, other terrace levels may be dated assuming constant rate of incision. 20. Rate of deformation XXX Dating assumes deformation rate constant over interval of concern and requires numerical dating for calibration. At spreading centers and plate boundaries, nearly constant rates may be valid for intervals of millions of years.
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Active Tectonics: Studies in Geophysics Method Applicability Age Range and Optimum Resolution Basis of Method and Remarks 102 103 104 105 106 21. Stratigraphy XXXX RESOLUTION DEPENDS ON RECOGNITION OF FEATURE AND ACCURACY OF DATING THAT FEATURE Based on physical properties and sequence of units, which includes superposition and inset relations. Depends on the establishment of time equivalence of units; deposition of Quaternary units normally occurs in response to cyclic climatic changes. 22. Tephrochronology X Requires volcanic ash (tephra) and unique chemical or petrographic identification and (or) dating of the ash. Very useful in correlation because an ash eruption represents a virtually instantaneous geologic event. 23. Paleomagnetism XX Depends on correlation of remnant magnetic vector, which includes polarity, or a sequence of vectors with a known chronology of magnetic variation. Subject to errors due to chemical magnetic overprinting and physical disturbance. 24. Fossils and artifacts XX Depends on the availability of fossils, including pollen, and artifacts. Resolution depends on the rate of evolution or change of organisms or cultures and on calibration by other techniques. Subject to errors due to misidentification and interpretation. 25. Stable isotopes X Depends on correlation of the sequence of isotopic changes with an age-controlled master chronology. Oxygen isotopic record is useful in deep-sea and ice-cap cores and perhaps in cave deposits. 26. Tektites and microtektites X Depends on recognition and dating of glassy material (tektites) formed during impact of extraterrestrial masses. Tektites are scattered over large areas, such as the Australo-Asian tektite field, formed about 700 ka. APPLICABILITY XXXX, nearly always applicable XX, often applicable XXX, very often applicable X, seldom applicable OPTIMUM RESOLUTION <2 percent 25–75 percent 2–8 percent 75–200 percent 8–25 percent tonism illustrate powerful applications of this method. Along the San Andreas Fault 55 km northeast of Los Angeles at Pallet Creek, about 50 carbon-14 ages date 11 episodes of faulting in the 1700 yr prior to the 1857 historical rupture and define an average recurrence interval of about 145 yr (Figure 13.2; Sieh, 1984). For the south Boso Peninsula of Japan, carbon-14 dating defines four stepwise uplifts of land relative to sea level in the last 7000 yr (Figure 13.3). Shimazaki and Nakata (1980) concluded that the offset history supports a time-predictable recurrence model; that is, the larger the amount of the coseismic slip, the longer the interval before the next earthquake (Figure 13.3). Carbon-14 ages may be in error by much more than the analytical uncertainty. Because of the extensive use of carbon-14 dating in studies of active tectonism, consideration should be given to the following three types of carbon-14 dating problems. Fluctuations in Atmospheric Carbon-14 Based on carbon-14 dating of tree rings whose absolute age is known, carbon-14 ages deviate from actual ages by amounts that are significant for some tectonic studies. For example, in the interval from 5000 to 8000 yr ago, carbon-14 ages are about 500–900 yr too young (Klein et al., 1982). In the late Holocene, fluctuations in atmospheric carbon-14 introduce significant uncertainty in dating tectonic events (Figure 13.4). For example, a carbon-14 age of 150+20 ya (years before AD 1950) only defines an age in the interval from 0 to 295 calendar years before AD 1950 (Klein et al., 1982, Table 2). Contamination with Old Carbon Independent of the age of the sample, the effect of contamination with old carbon is constant (Figure 13.5, upper left half). Regardless of whether a sample is 1 or 30 ka, incorporation of 10 percent “dead” carbon will make ages 800 yr too
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Active Tectonics: Studies in Geophysics FIGURE 13.2 Ages of faulting events (letters) on San Andreas Fault at Pallet Creek (from Sieh, 1984). Ages are based on about 50 carbon-14 dates and detailed trench mapping. old. Old carbon may contaminate a sample in two ways. First, detrital carbon, such as that from coal of pre-Quaternary age which contains no carbon-14 or that from humic soil material which may have an age of several thousand years, may be incorporated in a sample. Second, plants and animals living underwater can incorporate lower activity carbon from CO2 in the water either because the water is old or because it has bicarbonate from old rocks. For marine mollusks from the North Atlantic, the age of CO2 in ocean water increases ages by 400 to 750 yr (Mangerud and Gulleksen, 1975). Contamination with Young Carbon Contamination with recent carbon can alter ages greatly. Samples whose ages are beyond the range of carbon dating (>75 ka) that are contaminated with only half a percent of recent carbon will yield an age of about 40 ka (Figure 13.5, lower right half). Even the most exacting of analyses can be in error: a carbon-14 enrichment age of 71 ka (representing only 0.014 percent of the original carbon-14 activity) was thought to date the Salmon Springs glaciation of northwest Washington State (Stuiver et al., 1978) until associated ash deposits were fission-track dated at 700 and 800 ka (Easterbrook et al., 1981). Contamination with younger carbon may be responsible for many of the finite carbon-14 ages in the 20- to 70-ka range. Contamination of carbon samples with recent carbon produces effects that are not generally appreciated (Figure 13.5). Examination of this nonlinear effect also should provide a caution about assuming that “consistency” in age results necessarily is an argument for the validity of carbon-14 ages. For example, the consistency of many dates falling in the 25- to 40-ka range may only reflect contamination of samples older than about 50 ka with the equivalent of 0.5–2 percent of recent carbon (Figure 13.5). Several hundred carbon dates in the 25- to 40-ka range have been obtained from coastal deposits in the eastern United States. Many researchers (referenced in Bloom, 1983) have concluded that these samples date a mid-Wisconsin high stand of sea level, in part based on the apparent “consistency” of a large number of dates in this age range. After thorough analysis of this problem, Bloom (1983, pp. 215–218) concluded that these ages are invalid. Amino-acid racemization studies on mollusks asssociated with samples yielding caron-14 dates in the 25- to 40-ka range also indicate that the carbon-14 dates are erroneously young (Belknap, 1984). Samples may be contaminated by in situ additions of younger carbon. Soil carbonate, mollusks, or corals are particularly susceptible to addition of young carbon, especially when subject to repeated wetting or drying. Contamination by microorganisms incorporating young carbon may occur either before or after a sample has been collected. Marine cores stored for 5 yr were found to be contaminated by enough terrestrial bacteria to account for 5–10 percent contamination with modern carbon (Geyh et al., 1974). In-place contamination of samples is not well studied, but it may be possible if microbial activity consumes CO2 from air or from water with a higher carbon-14 activity than the age of the sample. For example, methanogens participate in terminal stages of the degradation of organic matter, living on and presumably incorporating carbon dioxide and hydrogen produced by anaerobic bacteria into their tissue (see Maugh, 1977). Extra care should be taken in order to minimize contamination with recent organic material during sampling and sample preparation. In addition, samples should be examined for visible contamination, particularly by roots. Removing modern roots will not, of course, remove contamination caused by older, largely decayed roots. Common sample pretreatment before carbon-14 dating removes base-soluble fulvic acids and acid-soluble humic acids, leaving a residue called
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Active Tectonics: Studies in Geophysics FIGURE 13.3 Four stepwise uplifts of the coast of the southern Boso Peninsula, Japan, determined by about 25 carbon-14 dates (from Shimazaki and Nakata, 1980). The open, half-closed, and closed circles indicate samples from above, near, and below the former sea level, respectively. Lower part of figure shows cumulative uplift versus time; line through corners of stairstep plot shows that uplift, inferred to be caused by coseismic faulting, conforms to a time-predictable model. FIGURE 13.4 Carbon-14 age as a function of dendrochronological age for the past 1000 yr (from Stuiver, 1982). Note deviations from concordance line that have a 100–200-yr period. humin or humate. Such pretreatment is commonly employed, but it does not remove all forms of contamination by younger carbon. For example, dating studies by Goh et al. (1977) have obtained ages as young as 17,750±2050 ya on humin from deposits whose “best” carbon-14 ages are more than 40,600 ya. Uranium-Series Dating The 230Th/234U disequilibrium method is the most commonly used dating technique of the many based on the radioactive decay series of uranium (Table 13.2; Ivanovich and Harmon, 1982). Because uranium is
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Active Tectonics: Studies in Geophysics FIGURE 13.5 Unequal effects of contamination with recent and dead carbon on carbon-14 ages (from Grootes, 1983). For contamination with dead carbon (shaded area), effect is a simple ratio. But for samples with true ages older than a few tens of thousands of years, the apparent age can be dictated by only small amounts of contamination with recent carbon. much more soluble than thorium, materials precipitated from solution such as corals, mollusks, calcic soils, carbonate deposits in caves and fault zones, and fossil bones are greatly enriched in uranium with respect to thorium. This provides a system in which radioactive growth of 230Th is a function of time until it reaches a steady-state relation with its parent 234U. The most reliable application of the method is to pure cave carbonate and to samples of corals in which the original aragonite is unaltered. For the time interval of 50 ka to about 300 ka, age control for deformation of coastal areas is largely based on uranium-series dating of corals. For corals and cave deposits, uranium is incorporated in the sample at the time of deposition and, under appropriate conditions, the sample acts as a closed system with respect to uranium. Dating problems generally result from migration of uranium after initial deposition or contamination with thorium in detrital material at the time of deposition. Living mollusk shells or bones contain only about a tenth of the uranium that becomes incorporated after burial. If ages are calculated on the basis of a closed system, the uranium in the sample should ideally have been introduced in a time interval that is short relative to the age of the sample, and none of this secondary uranium should have been leached away. If uranium is incorporated at later times in the history of the sample, the apparent Th/U age is too young. Conversely, if uranium is leached during the later history of a sample, the apparent Th/U age is too old. Kaufman et al. (1971) concluded that for samples of known age, 230Th/234U dating of mollusks gave ages within their analytical uncertainty no more than half the time. For active tectonism in semiarid and drier areas, 230Th/234U dating of carbonate may date fault movements. Soils may contain carbonate coats on the undersides of stones that can date faulted alluvial surfaces. Along the Arco fault scarp in central Idaho, a surface vertically offset about 19 m by faulting has 1-cm-thick carbonate coats that can be subdivided into outer, middle, and inner parts (Pierce, 1985). Analyses of both the soluble (carbonate) and insoluble (detrital residue) fractions from these layers yield ages as shown in Figure 13.6
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Active Tectonics: Studies in Geophysics FIGURE 13.6 Three stratigraphically consistent 230Th/234U ages from a stratified carbonate coat. This plot of age versus thickness defines an age of about 160 ka for an alluvial fan surface offset about 19 m along the same range-front fault, but 50 km south of the part offset during the 1983 Borah Peak earthquake, central Idaho. 230Th/234U isochron ages from Szabo and Rosholt (1982, Figure 10.5) are plotted at midpoint of sampled layer. The carbonate coats form on the underside of stones in the soil on an alluvial fan. (Szabo and Rosholt, 1982). These ages are stratigraphically consistent and indicate a time span of 160 ka for 19 m of offset and a long-term slip rate of about 1 m/ka. Potassium-Argon Dating The potassium-argon (K-Ar) method is limited in its applicability to dating active tectonics by its primary use with igneous rocks and general lack of resolving power for rocks younger than a few tens of thousands of years. Nevertheless, most of the correlation methods (Table 13.1, column 6) and calibration of many of the relative-dating methods (Table 13.1, columns 4 and 5) depend on K-Ar dating for their age control. More important to the study of active tectonism, both the paleomagnetic time scale, which was critical in establishing the theory of plate tectonics, and the relative rates of plate motions are largely based on K-Ar dating. Fission-Track Dating Fission-track dating is generally limited to rhyolitic volcanic rocks older than about 100 ka and is commonly better than the K-Ar method for dating volcanic ash. Fission-track dating can also be used in areas of high relief and rapid uplift to estimate long-term rates of uplift. Because fission tracks in apatite anneal if warmer than about 120°C, fission-track dating of apatite determines the time a rock has been below this temperature and consequently at relatively shallow depths. If the geothermal gradient is known, the rate of uplift and erosion of a mountainous area can thereby be determined. Using this method, Naeser et al. (1983) estimated that the late Cenozoic rate of uplift of the Wasatch Mountains has averaged 0.4 m/ka over the past 10 Ma (million years). OTHER RADIOLOGIC METHODS Other radiologic methods (Table 13.1, column 3) involve radioactive processes but may also depend on other nonradioactive processes that must be either estimated or accounted for by calibration using other numerical methods. Thermoluminescence (TL) and Electron-Spin-Resonance (ESR) Dating Thermoluminescence (TL) dating (Wintle and Huntley, 1982) may be widely applicable to dating active tectonics, but the method has been used rarely in the United States. Owing to the effects of radiation, many minerals emit light when progressively heated, and the changing intensity of light emission yields a glow curve. A starting point for buildup of TL can be the time of crystallization of carbonate in a soil, the growth of a shell, the firing of a ceramic, the eruption of a lava flow, or the time of burial after exposure to sunlight. The glow curve becomes stronger the greater and longer the radiation exposure. The flux of radiation is related to the modern content of uranium, thorium, and potassium. The response of the dosimeter grains is calibrated by artificial irradiation (alpha, beta, and gamma) and measurement of the induced glow curves. Several hours of exposure to sunlight “zeros” the TL in grains of quartz and feldspar, which are nearly ubiquitous in surficial deposits. Thus, many surficial deposits offset by faulting are potentially datable by TL. Because water adsorbs some of the gamma radiation, the amount of water present over the history of the sample needs to be measured or estimated and its effect on the in situ sample calculated. Also, radiogenic isotopes
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Active Tectonics: Studies in Geophysics are assumed not to have been added to or leached from the sample. Wintle and Huntley (1982) listed criteria for evaluating TL dates of sediment and discussed problems that merit further research. TL dating of loesses has yielded reasonable results. Late Pleistocene loess in Germany yielded TL ages that are stratigraphically consistent and concordant with carbon-14 ages of an associated ash (Wintle and Brunnacker, 1982; Figure 13.7). Electron spin resonance (ESR) dating is similar to TL dating in that a mineral acts as a dosimeter for local ra FIGURE 13.7 Thermoluminescence (TL) dating of loess deposits in West Germany showing that the TL ages are both stratigraphically consistent and concordant with existing carbon-14 ages on the volcanic ash that was dated elsewhere (redrawn from Wintle and Brunnacker, 1982). diation, but differs in the feature measured and the method of measurement. ESR dating of corals from marine terraces of Japan yielded results in good agreement with carbon-14 (2–4 ka) and 230Th/234U (40–>200 ka) dating (Ikeya and Ohmura, 1983). Uranium-Trend Dating Uranium-trend dating is an isochron-type method for dating Quaternary sediments and soils (Rosholt, 1980; Szabo and Rosholt, 1982). Rather than requiring a closed system as do most uranium-series methods, this method depends on a flux of uranium (mostly 238U) through sediments and the consequent embedding of recoil products 234U and 230Th in the sediments. The method can be used on almost all sedimentary materials and has produced generally reasonable ages (Rosholt, 1980). The calibration of the method initially depended on ages determined by other methods, but ages can now be calculated using the established calibration. The method is expensive, requiring determination of 238U, 234U, 230Th, and 232Th for five or more samples from a given deposit. Although widely applicable, the method has so far experienced limited use because of its newness, complexity, and cost. Cosmogenic Isotopes Other Than Carbon-14 Similar to carbon-14, several other radiogenic isotopes are generated by cosmic-ray bombardment and may be useful in tectonic studies. Dating rationales exist for 32Si, 41Ca, 36Cl, 26Al, 10Be, 129I, and 53Mn, which are analyzed by accelerator mass spectrometry (half-lives given in Table 13.2). Beryllium-10 is produced by cosmic-ray bombardment and is carried to the Earth’s surface by rain and dust. Pavich et al. (1984) showed that 10Be is adsorbed onto clays in soils and systematically increases in abundance with soil age for at least the first 100 ka of soil development. More research is needed to evaluate this method for dating surface and buried soils. RELATIVE-DATING METHODS, SIMPLE PROCESS These methods (Table 13.1, column 4) do not depend on radiogenic processes but are based on relatively simple chemical or biological processes whose rates are related to controlling variables such as temperature and chemical composition or species effects.
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Active Tectonics: Studies in Geophysics Obsidian-Hydration Dating Obsidian hydration has been used to date the last two glaciations in the Rocky Mountain region (Pierce et al., 1976). If temperature and chemical composition are constant, hydration thickness increases proportional to the square root of time. Figure 13.8 shows the increase in hydration with time as determined by the hydration thicknesses of two K-Ar-dated rhyolite flows and carbon-14-dated recessional deposits. The deposits of the next to last, or Bull Lake, glaciation date at about 140 ka (Pierce et al., 1976). This age for the Bull Lake glaciation is about 5 times older than the age considered correct 20 yr ago. The importance of the dating at West Yellowstone to studies of active tectonism is that these ages can be inferred for many deposits throughout the western United States if they can be correlated with FIGURE 13.8 Obsidian-hydration dating of the Pinedale and Bull Lake Glaciations near West Yellowstone, Montana (from Pierce et al., 1976). Dashed line shows increase in hydration thickness with age based on hydration thickness measured on deposits dated by K-Ar or 14C methods. Above histograms, short lines with dots are means and standard deviations of hydration-thickness measurements, some of which are offset (small arrows) to account for small temperature differences between localities. those at West Yellowstone on the basis of criteria such as soil development, morphologic changes, and weathering rinds (see Figure 13.9). Amino Acid Racemization Amino acid racemization (and epimerization) has provided important age information for deciphering late Quaternary deformation along the West Coast of the United States (Lajoie, Chapter 6, this volume). Because racemization rates for a given species depend on temperature and a kinetic model (Wehmiller, 1982), the method works best if calibrated by numerical methods. On the California coast, uranium-series dating of corals has provided a few calibration points, but even with this calibration the amino acid ratios on mollusk samples did not allow distinction between three global sea-level culminations known from elsewhere to date at about 80, 95, and 125 ka. The problem of distinguishing these three high-sea stands has been resolved by combined studies using amino acid and uranium-series dating, temperature gradients along the coast, and paleontological identification of cool (oxygen isotope substages 5a or b) and warm (substage 5e) faunas (Lajoie, Chapter 6, this volume). RELATIVE-DATING METHODS, COMPLEX PROCESSES This group of dating methods includes some of the most widely applicable methods (Table 13.1, column 5). Numerical ages can be empirically estimated by these methods. Rigorous evaluation of these complex methods would require modeling of each process and quantification of their relative effects. Nevertheless empirical quantification has been done, and some age estimates based on these methods (Table 13.1, column 4) may be more reliable than, if not so precise as, some carbon-14 ages. Rock and Mineral Weathering Rock and mineral weathering (Table 13.1, column 5) includes such relative-dating techniques as mineral grain etching, seismic velocities in weathered stones, pitting on stone surfaces, and weathering rinds. Weathering rinds on basaltic and andesitic stones from the B horizons of soils have yielded age information on middle and late Quaternary deposits at seven different areas in the western United States (Figure 13.9; Colman and Pierce, 1981). Multiple measurements of rind thicknesses from a given stratigraphic unit are consistent; and, for a succession of deposits, rind
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Active Tectonics: Studies in Geophysics FIGURE 13.9 Use of weathering rinds to date and correlate glacial deposits in seven areas in the western United States. As shown by decreasing curvature of lines, rind thickness is assumed to increase logarithmically with time (from Colman and Pierce, 1984). Open circles are for deposits independently dated at West Yellowstone and for deposits correlated on basis of soils and other criteria with 140,000-yr-old deposits at West Yellowstone. Solid circles are for other glacial deposits plotted on the appropriate curve according to their rind thickness. thickness increases with stratigraphic age. Local calibration by numerical dating indicates that rind thicknesses increase logarithmically with time. Based on a logarithmic increase in rind thickness and on the assumption that deposits with a certain relative moraine form and degree of soil development correlate with deposits of the Bull Lake glaciation at West Yellowstone (oxygen-isotope stage 6), ages can be estimated for all the deposits in seven glacial successions in the western United States. Deposits representing isotope stages 2 and 6 apparently are present in all areas sampled, but moraines representing stages 3 and 4 were apparently obliterated in many areas by glaciation during stage 2 (Figure 13.9). Recent developments in the study of desert varnish suggest that systematic changes in varnish properties, such as decreasing ratios of leachable cations to manganese, occur with time (Dorn, 1983). With local calibration, age estimates on faults in desert environments may be obtained by this method. Soil Development On land, soil development is nearly always pertinent to estimating the age of deformation. Soil development is a function of climate, parent material, organisms, topography, and time. If all the factors other than time can be held constant, the effect of time on soil development can be isolated and used to calibrate soil development with time at other sites. Recently, Harden (1982) devised the soil “Profile Development Index” based on quantification of standard field descriptions of soils, including such features as color, clay content, texture, and soil-horizon thickness. Each of 10 or so soil properties is objectively quantified for each soil horizon on a scale that goes from zero to the maximum observed development. For the Merced, California, area, an individual soil property such as rubification (reddening and brightening of soil colors) shows a progressive increase with time from 100 yr to more than 1 m.y. (rubification, Figure 13.10A). The Profile Development Index (Figure 13.10B) combines several soil properties such as texture, pH, dry consistence, and soil structure and shows the cumulative effect of the development of many soil properties with time. Dating by this soil Profile Development Index is improved by using only soil properties that show the highest correlation with age (see Index of four best properties, Harden and Taylor, 1983). Although calibration of soil Profile Development Index with age is best restricted to local areas where climate and parent material are the same, soils from four different areas of the United States appear to show similar Profile Development Index values with increasing age (Figure 13.10C). The soil Profile Development Index should prove useful in estimating ages of deformation, for it is based on readily describable field properties, provides an objective numerical basis for comparison between soils, and eliminates the need for subjective estimates of a soil’s “development.”
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Active Tectonics: Studies in Geophysics FIGURE 13.10 Increase in soil development with time (from Harden, 1982; Harden and Taylor, 1983). A, one soil property (rubification) in Merced, California, area. B, Soil Profile Index based on multiple properties in Merced area. C, Soil Profile Index for climatically different areas. Numbers indicate the number of points that plot in one place. Some individual soil properties can be measured to estimate age. Such properties include increases in clay (Levine and Ciolkosz, 1983; Reheis, 1984; Pierce, 1979), secondary carbonate (Machette, 1978), secondary gypsum (Reheis, 1984), and secondary iron oxides (McFadden, 1982) as well as major-element chemistry (Harden and Taylor, 1983, Reheis, 1984). These measurements of changes are for an individual process and are like the simple-process, relative-dating methods (Table 13.1, column 4) but are discussed here because they are a component of soil development. On the downwind and downthrown side of a fault offsetting a flat, former basin floor of the Rio Grande rift, New Mexico, fault movements were rapidly followed by deposition of eolian sand. During stable periods, five different calcic soils developed on these eolian sands and were subsequently buried (Figure 13.11). The time taken to form each soil is based on the total pedogenic CaCO3 (g/cm2-soil column) divided by the accumulation rate of pedogenic CaCO3 determined from the 500-ka surface soil on the upthrown side of the fault. The deposit thickness indicates fault offset, whereas the amount of pedogenic CaCO3 indicates the interval between fault episodes (Figure 13.11). The deformation history inferred from this information (Figure 13.12) shows an apparently decelerating rate of faulting. Progressive Landform Modification Recognition of active tectonism commonly depends on detection of landforms created or modified by deformation, which is the subject area of tectonic geomorphology. To the trained eye, tectonic landforms tell much about the degree of tectonic activity, and new quantitative methods are making tectonic geomorphology a more exact science (Keller, Chapter 8, this volume). The effects may be subtle, such as alterations of river courses and gradients (Schumm, Chapter 5, this volume) or dramatic, such as bold, mountain-front escarpments (Keller, Chapter 8, this volume). Geomorphic modification of fault scarps is particularly important to studies of active tectonism (Wallace, 1977; Nash, Chapter 12, this volume; Keller, Chapter 8, this volume). As with soils, the environmental variables such as lithology, climate, and vegetation need to be held constant or accounted for otherwise.
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Active Tectonics: Studies in Geophysics FIGURE 13.11 Dating of fault-related buried soils based on the accumulation of carbonate in calcic soils near Albuquerque, New Mexico (from Machette, 1978). CORRELATION METHODS If a feature can be correlated with an event of known age, reliable and precise age control can be obtained. Methods such as those listed in Table 13.1 (column 6) may provide accurate numerical dates or exact correlation between deformed areas. Stratigraphy Stratigraphy, including lithologic characteristics and the sequence of units, is basic to understanding the history of active tectonism. In surficial geology, the sequence of units may not be based on superposition but on geomorphic relations such as a sequence of successively lower and younger stream terraces. The origin of deposits also may be important in understanding the stratigraphy of surficial deposits. If the age of a unit can be determined in one place, that age can be applied to correlative units or be used to provide age constraints for sequentially younger or older units. Many surficial geologic units are causally related to the cycles of climatic change that characterize the Quaternary, such as successions of glacial till, sequences of loess separated by buried soils, and sequences of marine or alluvial terraces. Numerical dating control is normally obtainable only at scattered localities, and extension of this dating control to sites of deformation depends on stratigraphic correlation over distances of tens to even hundreds of kilometers. The age of stratigraphic units in Quaternary geologic successions is the subject of much current research. Such research addresses many unresolved dating problems, results in new and commonly significantly revised ages, and leads to development of new strategies for obtaining ages. Studies centered on stratigraphy offer the best method to check a given dating technique through comparison with other age information from related stratigraphic units. Thus, although laboratory and numerical analyses are important in obtaining ages, stratigraphic work based on field studies is fundamental FIGURE 13.12 Fault offset versus time as determined by accumulation of soil carbonate in buried soils near Albuquerque, New Mexico (drawn from data in Machette, 1978).
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Active Tectonics: Studies in Geophysics in judging the reliability of these ages and relating ages to deformation of stratigraphic units. Tephrochronology A volcanic ash can provide a time-parallel marker whose age is as accurate as the best dating either at any of its occurrences or of the correlative volcanic rocks in its source area. Recognition of a given ash bed should be based on multiple criteria, especially the petrography and chemistry of the glass and phenocrysts, as well as stratigraphy, paleomagnetism, paleontology, and radiometric dating (Westgate and Gorton, 1981). Tephrochronology has proved of great value in dating active tectonics. For example, in southern California deposits containing the 0.7-Ma Bishop ash are laterally offset about 6.6 km on the San Jacinto Fault (Sharp, 1981), and deposits containing 0.6-, 0.7-, and 1.2-Ma volcanic ashes are uplifted and tilted along the Ventura Fault (Yeats, Chapter 4, this volume). Tephrochronology is also important in calibrating other relative-dating and correlation methods, such as soil development, uranium-trend dating, amino-acid racemization, thermoluminescence dating, and dating of faunal boundaries. These calibrated methods can then, in turn, be used to date active tectonism. Improvements in tephrochronology have led to major revisions in Quaternary stratigraphy and age assignments. In the 1960s, the Pearlette ash (known present distribution from California to Iowa) was considered to be of a single age, and the stratigraphy and paleontology of older Quaternary deposits all the way from the midcontinent to the Rocky Mountains were founded on the assumption that the Pearlette ash represented one eruption of late Kansan age. Careful petrographic and chemical study of the Pearlette ash has shown that it actually includes ashes from three different and distinct eruptions from the Yellowstone area that differ in age by a factor of 3 (Huckleberry Ridge ash, 2 Ma; Mesa Falls ash, 1.2 Ma; and Lava Creek ash, 0.6 Ma; Izett, 1981). Paleomagnetism The orientation of the Earth’s magnetic field is recorded by the orientation of magnetic minerals at the time of deposition of many fine-grained sedimentary deposits. Dating control can be obtained if the paleomagnetic record determined from a sequence of late Cenozoic sediments or volcanic rocks can be correlated with the established paleomagnetic polarity time scale (Mankinen and Dalrymple, 1979; see Barendregt, 1981, for a review). For example, the change from the Matuyama Reversed-Polarity Chron to the present Brunhes Normal-Polarity Chron occurred about 730 ka. This change provides a global datum for the assessment of long-term tectonic rates and calibration of relative-dating methods. The established polarity time scale also dates the age of the ocean floor outward from the ocean-ridge spreading centers. The rates of movement of crustal plates away from these spreading centers is based on the age and width of the normal and reversed polarity stripes of the ocean floors. Within the Brunhes Normal-Polarity Chron, potential age control may be provided excursions or reversed-polarity subchrons that lasted a few thousand years; about five such events have been suggested. In addition, secular variation in the geomagnetic field with periodicities of thousands to tens of thousands of years may provide a basis for local correlation and dating. The record of secular variation is best studied in lacustrine and other environments of continuous fine-grained sediment deposition. Such sediment dated by paleomagnetic criteria may also record sediment deformation associated with nearby earthquakes. Fossils and Artifacts Fossils have been of limited value in dating young deposits because the amount of Quaternary evolutionary change has been small. Some organisms, such as the rapidly evolving microtine rodents, do show several changes each million years and can therefore be of great value in estimating long-term deformation rates. An example of dating active tectonics comes from east of San Francisco Bay where the Verona Fault offsets Livermore Gravel and is mapped within 60 m of the General Electric Test Reactor at Vallecitos (Herd, 1977). The age of this faulted gravel was poorly known until it was dated using small mammal faunas as about 500 ka (C.A.Repenning, U.S. Geological Survey, personnal communication, 1984). The cyclic climatic changes of the Quaternary Period resulted in cyclic changes in plant and animal populations. Such plant or animal changes also provide a basis for dating active tectonism. For example, pollen assemblages representing climate considerably colder than at present may be used to infer a pre-Holocene age (>10 ka). WHY DATING SPANNING DIFFERENT TIME INTERVALS IS NEEDED Geologic prediction of future deformation requires enough dating control to understand if and how deformation has changed through time. For most active tec-
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Active Tectonics: Studies in Geophysics tonism, we know little about whether strain rates are uniform through time. Even with the simplifying assumption that strain and long-term slip rates are uniform, a fault scarp with evidence of recent movement can yield dramatically different predictions: (1) if the slip rate is fast, future movements are likely soon, or (2) if the slip rate is slow, future movements are unlikely soon. Spreading rates at oceanic ridges and movement rates of crustal plates appear to be rather constant over the planning intervals of concern to man and his activities. Consequently, constant strain rates may be appropriate for some major tectonic features. For many individual faults, however, this assumption is probably not valid. Even for some faults in the boundary zone between crustal plates, slip rates have changed greatly. On the San Jacinto Fault southeast of Los Angeles, the slip rate for the past 730 ka has averaged about 9 mm/yr, whereas the rate between 0.4 and 6 ka averaged about one-fifth that (Table 13.3; Sharp, 1981). These large differences in rate may have resulted from differential movements between the Pacific and American plates being localized at times on the San Jacinto Fault and at other times on the nearby San Andreas Fault (Sharp, 1981). On a fault in the Rio Grande rift near Albuquerque, New Mexico, the rates of deformation have decelerated over the last 400 ka. Recurrence intervals on this fault are quite long, averaging more than 100 ka (Figure 13.12 ). For the total Basin and Range province encompassing the 700-km distance between the crests of the Wasatch Mountains of Utah and the Sierra Nevada of California, the overall rate of extension may be relatively constant. But Holocene and historical activity in the Basin and Range is spatially clustered in zones. Holocene tectonic events (Ms about 7 or larger) define an eastern and western zone; these zones are separated by a zone about 300-km wide encompassing the Nevada-Utah border in which no late Quaternary scarps are recognized (Wallace, 1981). Grouping of events in time may also occur. Some segments of the Wasatch Fault zone have had three or more TABLE 13.3 Variable Slip Rates Through Time, San Jacinto Fault, Southern California (from Sharp, 1981) Time Interval Slip Rate (mm/yr) Change in Slip Rate Through Time 0–400 3.9±1.1 Twofold increase 400–6000 1.7±0.3 Fivefold decrease 0–730,000 Holocene (last 10 ka) offsets and exhibit slip rates during the Holocene of 1.3+0.1 m/ka, yet other sections of the Wasatch Fault zone have not been active in the Holocene (Schwartz et al., 1983). In addition, based on fission-track annealing ages, the uplift rate of the Wasatch Mountains for the past 10 Ma has been about one-third the Holocene rate, or about 0.4 m/ka (Naeser et al., 1983). Work in progress on deposits as old as 250 ka sheds light on the meaning of these rates (Machette, 1984). As dated by calcic-soil development, the slip rate has been on the order of 1 m/ka during the last 5 ka; this rate appears to have been more than 5 times greater than that over the past 250 ka, suggesting variable slip rates and temporal grouping of fault offsets (Machette, 1984). Rates of fault slip or other deformation are dependent on both the deformation pattern through time and the time window of observation. To illustrate this point, Figure 13.13 shows the effect of short, medium, and long time windows on the slip rates for five patterns of deformation: accelerating, constant, decelerating, episodic-quiescent, and episodic-active. For convenience, the deformation patterns are shown as systematic, and the deformation rates are arbitrarily set at 1 for the longest interval. Depending on the time window, deformation rates for each pattern may differ by more than an order of magnitude. Also, although the long-term rate is arbitrarily set at 1, the short interval has rates that differ by more than 2 orders of magnitude (Figure 13.13B), Deformation rates determined for differing time intervals will contribute to an understanding of deformation patterns through time in different tectonic settings. Few fault histories such as those shown by Figures 13.2, 13.3, and 13.12 have been determined. Deformation histories like that shown by lines for episodic-active and episodic-quiescent (Figure 13.13) have not been well documented, but, as discussed previously, some evidence suggests that these patterns of deformation have occurred. Only by understanding the history of a fault can we better understand what can be expected in the future. Consequently, the deformation history needs to be defined by multiple dates. The simplifying assumption that tectonic events such as rates of faulting or rates of uplift are constant may be useful as a first approximation, but this assumption may be quite misleading in some tectonic environments. Few fault histories are well enough dated to know in what cases the assumption of constant slip rate is valid and in what cases it is not. By detailed documentation, we can construct predictive models appropriate to a given tectonic setting. If movements on a given fault are grouped in time, or if faults in an area alternate in activity, concepts such as constant
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Active Tectonics: Studies in Geophysics FIGURE 13.13 A, five patterns of fault offset versus time. B, contrasting slip rates for these five patterns over short, medium, and long intervals of time. Slip rates are in arbitrary distance/time units, and long-term deformation for all patterns has been arbitrarily set at 1. slip rate or constant recurrence intervals would be, of course, quite misleading. Thus, sufficient dating related to deformation histories is required to understand the character of faulting in different tectonic settings and thereby to anticipate more intelligently the future deformation over time intervals of concern to man. Dating of a single fault history without concern for related faults may be examining too small a component, for relatively constant tectonic activity may be unevenly distributed among a group of faults. The definition of rates and knowledge of the constancy or variation of these rates through time permit quantitative ranking of tectonic activity, both for the purposes of scientific understanding and hazard assessment. DATING METHODS IN FUTURE RESEARCH ON ACTIVE TECTONISM All the dating methods discussed (Tables 13.1 and 13.2) have importance to studies of active tectonism. In the last decade, much improvement, refinement, and evaluation of the reliability of these methods have been associated with studies of active tectonism. Because experimental methods such as 36Cl and 26Al are numerical and based on radioactive decay, some may consider such methods the most promising for future advances. But such methods also involve assumptions concerning nonradiometric processes—one assumes that the isotope measured both accumulated at a known rate and no subsequent leaching has occurred. These nonradiometric factors are difficult to evaluate rigorously and have similarities with, for example, the extremely complex process of soil development. For example, 10Be accumulation in soils is influenced by the clays in the soil, which generally increase in quality and may change mineralogically as the soil develops. The recent quantification of soil development using the Profile Development Index, as well as analyses of changes in individual soil components such as soil carbonate or clay, can provide useful but not precise dating control; soil development is nearly always applicable to the dating of active tectonism.
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Active Tectonics: Studies in Geophysics Because of their nearly universal applicability, thermoluminescence and uranium-trend dating of sediments are promising methods for dating deformation, especially in the 104- to 106-yr range. Compared to this potential, research by laboratories on these methods and application to active tectonics have been limited. Carbon-14 dating is the most important method in the range of about 30,000 yr to the present. Research using current methods of organic chemistry may help to resolve problems with the method and the mechanisms of contamination. Even before sample collection, microbes may have lived in organic material and incorporated younger carbon from the surrounding water or air. Important carbon-14 samples might be quantitatively examined for such microbes and other potential contaminants to estimate the importance of their effect, as was done by Geyh et al. (1974). Carbon-14 dating using accelerator mass spectrometry permits the dating of milligram-sized samples (Grootes, 1983). If only small samples are present, this offers obvious advantages, but contamination within, for example, an individual grain of charcoal, may still be present. Special organic concentrates in combination with accelerator dating may offer real improvements. For example, a whale bone sample from northern Alaska has been carbon-14 dated, but considerable question remains about the reliability of the ages (D.L. Carter, USGS, personnal communication, 1984). Because one amino acid is common in bone but not in the likely contaminating materials, dating of this aminoacid sample may provide a reliable carbon-14 age. The amino acid concentrate will be small, requiring dating by accelerator mass spectrometry. In the quest for better dating of active tectonism, the importance of local, time-calibrated stratigraphies should not be underestimated. Datable materials are commonly not found where a given stratigraphic unit is offset but may be found elsewhere in that stratigraphic unit. Because the stratigraphy of many Quaternary deposits reflects the cycles of climatic changes, the ages of faulted or uplifted datums can be inferred if relations between stratigraphic units and climatic cycles are known. Study of active tectonism and Quaternary stratigraphy should proceed together also because evidence of tectonism and climatic change is commonly similar. For example, gravel deposition may result from uplift, from climatic change, or from both uplift and climatic change. In conclusion, knowledge of active tectonism of a given area progresses as both the amount and age of deformation are determined. Such knowledge commonly develops by documenting first the age and amount of the most recent movement; second, the history of the last several movements; third, deformation over contrasting time intervals, including long intervals; and last, the relation between the dated deformations on associated faults. Dating control is commonly the limiting factor in understanding active tectonism. Normally, several dating methods need to be used because of (1) the limited range of a given technique, (2) the presence of appropriate materials, and (3) the need to check the reliability of any given dating technique by using another method. For studies of a fault segment, for example, dating control for Holocene activity can be determined by carbon-14, and studies of surface and buried soils can be made to provide dating control in the 104- to 105-yr range as well as to provide a check on the reasonableness of the carbon-14 dates. Tables 13.1 and 13.2 list 26 methods. Development or refinement of the methods, particularly those listed in columns 3 to 6 of Table 13.1, has advanced greatly in the past decade, in part because of the impetus to date active tectonics. With continued effort, comparable advances are possible in the future. REFERENCES Allen, C.R. (1975). Geologic criteria for evaluating seismicity, Geol. Soc. Am. Bull. 86, 1041–1057. Barendregt, R.W. (1981). Dating methods of Pleistocene deposits and their problems: IV, Paleomagnetism, Geosci. Canada 8(2), 56–64. Belknap, D.F. (1984). Amino-acid racemization in the “mid-Wisconsin” C-14 dated formations on the Atlantic Coastal Plain, Geol. Soc. Am. Abstr. Progr. 16, 2–3. Bloom, A.L. (1983). Sea level and coastal morphology of the United Statesthrough the Late Wisconsin glacial maximum, in The Late Pleistocene, S.C.Porter, ed. (Volume 1 of Late Quaternary Environments of the United States, H.E.Wright, ed.), University of Minnesota Press, Minneapolis, pp. 215–229. Bonilla, M.G. (1967). 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Representative terms from entire chapter: